One well known and cheap solution consists in making the rim out of "segments" of thin magnetic sheet material. Each segment occupies an angular sector of a flat ring, and the segments are obtained by punching out the required shape in a press. The segments are then stacked and juxtaposed to make up an axial succession of complete rings around an axial portion of the rotor. The number of segments round a ring and the angular extents thereof (the segmentation) are determined as a function of the number of pole pieces and of the maximum size of segment that can be punched from the feed stock (may be set by maximum available punch size, by the width of the feed sheet, or by the percentage of waste material that can be accepted). The segments are clamped axially between thicker end segments by means of many tie rods. Providing the completed rotor is transportable to the end site, the rim may be assembled in the factory, otherwise the rotor must be assembled on site.
The resulting rim comprises a large number of lamination layers which are usually 2 to 4 mm thick. Each lamination layer is in the form of a ring having a plurality of segments, each of which extends over an angle corresponding to an integer number of pole pieces (see FIG. 1).
The segments making up the individual rings may comprise single laminations or else a small number of laminations stacked together, but in either case, axially adjacent segments are offset from one another through an angle which is equal to one or more pole pitch steps, or in some case two to one or more half pole pitch steps. The offset is provided to give mechanical strength to the assembly as explained below. The relative angular displacement between axially adjacent segments is refered to herein as the "segment overlap angle".
The inductive pole pieces are fixed to the periphery of the rim by means of T-shaped, dovetailed or otherwise suitably shaped keys. Each pole piece comprises one or more keys. The axes of the keys are all parallel to the axis of the pole piece.
The rim built up in this manner is tubular in form, and when the rotor rotates, the main stress to which it is subjected is tension in the circumferential or tangential direction. This is due to the effects of centrifugal forces acting on the rim itself and also on the pole pieces attached to it.
The radial thickness of the rim determines the cross section of the metal which must withstand said stress. However, the presence of radial gaps between adjacent segments in the same ring make it locally impossible for the rings to transmit said tension. The rim as a whole does not fall apart because the forces are transmitted across the gaps by the combination of two effects: shear in the tie rods; and friction between axially adjacent rings which enables across-gap force to be transmitted through the adjacent rings, provided the rings are offset in such a way that the gaps do not line up. It will thus be understood that when calculations are performed to determine the maximum strength of the rim, these calculations are based on the axial section of the rim which includes the largest possible number of gaps, i.e., on a "plane of weakness". In FIG. 2 which shows the outer surface of a prior art rim developed in the plane of the figure, a plane of weakness is shown as a line P1. The gaps are shown at F. Compared with a rim that does not have any gaps, the useful cross section of metal in the plane of weakness is reduced by the ratio (p-1)/p, where p is the ratio of the angular extent of the segments by the minimum possible segment overlap angle. In known rims the ratio p is equal to the number of pole pitch steps per segment, or sometimes to the number of half pole pitch steps per segment.
The ratio (p-1)/p is referred to herein as the "overlap factor". Usually, in large synchronous machines where segmented rims are commonly employed, the number of pole pitch steps per segment is 2, 3, 4 or rarely 5. The corresponding values of the overlap factor are 0.50, 0.66, 0.75 and 0.80.
It can thus be seen that the mechanical size of a rim of this type is directly influenced by the overlap factor, and the feasibility limits for high speed machines lead to segmented rims being replaces by non-segmented or "solid" rims in which each ring is made from a single piece of forged or cast steel.
Such solid rims have the following disadvantages:
(1) they are expensive to manufacture;
(2) the reject rate because of internal defects in the rings can be quite high, thus lengthening the time taken to manufacture the rim;
(3) large scale machining equipment is required; and
(4) the size of the parts may lead to transport difficulties.
Preferred embodiments of the present invention provide a rim made up of segments which are stamped out of a sheet of magnetic material and which are assembled substantially as described, however, by virtue of an increased overlap factor, e.g. 0.90 or more, they enable this type of construction to be used at speeds which previously necessitated solid rims.